Blood coagulation is a delicately regulated process that serves as a protective mechanism against blood loss due to tissue damage. Overactive or unregulated coagulation can lead to conditions including myocardial infarction, stroke, deep vein thrombosis (DVT), and pulmonary embolism. The ability to recognize coagulation disorders and quantify their severity is critical for identifying those at risk and implementing appropriate prophylactic treatment. Because of inherent risks accompanying anticoagulant therapy, such as hemorrhage or anaphylaxis, it is critical that such therapies be prescribed appropriately (see Anderson et al., “Best Practices: Preventing Deep Vein Thrombosis and Pulmonary Embolism”, Center for Outcomes Research, U. Mass. Med. Ctr, 1998.
Hypercoagulability, or thrombophilia, is an inherited or acquired coagulation disorder in which there is either an overactivation of coagulation or deficient deactivation of developed thrombus. While a number of factors within the coagulation cascade such as factor V Leiden, protein C or S deficiency, and antithrombin III deficiency are known to increase the propensity to clot (see Harris et al. “Evaluation of Recurrent Thrombosis and Hypercoagulability”, American Family Physician, vol. 56 (6), Oct. 15, 1997, there is currently a dearth of techniques available to quantify these effects clinically. The methods currently available are mostly biochemical in nature and test for a specific genetic mutation or abnormal chemical reaction rate, such as Leiden Factor V. mutation R560Q; Hyperhomocysteinemia MTHFR Mutation; Prothrombin Gene Mutation 20210; Protein C levels; Protein S levels; Activated Protein C activity; antibodies to six phospholipids of the IgM, IgG and IgA classes; Lupus anticoagulant antibody; Russell Viper Venom time; Activated Partial Thromboplastin time; and Prothrombin time. While these tests may provide valuable information, they are unable to determine the coagulation rate of an individual's blood. Furthermore, since the coagulation cascade is exceedingly complex, there are numerous steps in the pathway that might be disrupted or inappropriately regulated. However, it is not always possible to determine if these interruptions in the cascade are predictive of an observable clinical impact on thrombus formation.
Mechanical methods, such as cone and plate viscometry or indentation testing, provide the most intuitive way to characterize the mechanical parameters of blood coagulation. However, these approaches are limited because the mechanical forces applied to the forming thrombus can disrupt its delicate structure, and thus disturb the system enough to interrupt the normal course of coagulation.
Deep vein thrombosis (DVT) refers to the formation of a blood clot in a large vein of the leg. DVT often results from a lack of movement in the extremities for significant periods of time or from an increased propensity to clot due to malignancy, recent surgery or trauma, pregnancy, hormonal agents such as oral contraceptives, or other contributing causes, see Hirsh et al., “How We Diagnose and Treat Deep Vein Thrombosis”, Blood, vol. 99(1), pp. 3102-3110. If a portion of the thrombus breaks off and travels to the pulmonary vessels, a potentially fatal pulmonary embolism can result. Clinical diagnosis cannot serve as the sole means of DVT diagnosis because many potentially dangerous venous thrombi are asymptomatic, and many of the symptoms are not unique to DVT. Current noninvasive methods of diagnosis such as duplex ultrasonography, venography, impedance plethysmography, and MRI can often detect the presence of a clot, but are limited by an inability to determine the stage of development of the clot so identified. Furthermore, these methods must often be used in conjunction with another diagnostic method or tool such as the d-dimer assay in order to make a conclusive diagnosis.
Although duplex ultrasonography is favored for the initial investigation of DVT, several groups have also proposed the use of ultrasound to extrapolate parameters related to the formation of DVT. Shung et al, in “Ultrasonic Characterization of Blood During Coagulation”, Journal of Clinical Ultrasound, vol. 12, pp 147-153, 1984, have shown that the increase in echogenicity associated with the formation of a thrombus is mostly due to an increase in ultrasonic backscatter. They have also found increases in both the attenuation coefficient and the speed of sound. Parsons et al. in “Age Determination of Experimental Venous Thrombi by Ultrasonic Tissue Characterization”, Journal of Vascular Surgery, vol. 17(3), pp. 470-478, 1993 (which is hereby incorporated herein, in its entirety, by reference thereto), have been able to differentiate in vivo between clots of varying ages by looking a the slope and intercept of the linear fit of the normalized power spectrum. Emelianov et al., in “Ultrasound Elasticity Imaging of Deep Vein Thrombosis” Proc. IEEE Ultrasonic Symposium, 2000 (which is hereby incorporated herein, in its entirety, by reference thereto), have characterized different clinical stages of a thrombus using maps of local strain. Their method operates by obtaining baseline radio frequency (RF) echo data, mechanically compressing the tissue, obtaining a second compressed set of data, and applying signal processing methods to create maps of local strain. Rubin et al., in “Clinical application of sonographic elasticity imaging for aging of deep venous thrombosis: preliminary findings,” Journal of Ultrasound in Medicine, vol. 22, pp. 443-8, 2003, which is incorporated herein, in its entirety, by reference thereto, characterizes different clinical stage of a thrombus using maps of local strain obtained by compressive elastography. Although the techniques proposed by Parsons et al., Emelianov et al. and Rubin et al. have yielded valuable results, they are primarily focused on age classification of DVT and thus are not able to characterize thrombus formation. Furthermore, these techniques do not provide information about coagulability and are thus of little or no value in prospectively identifying patients at high risk of forming a blood clot. Furthermore, direct translation of these techniques to benchtop tools is problematic because of high variability in measurements taken.
There remains a need for the ability to characterize changes in soft tissue, and particularly for characterizing thrombus formation. There remain needs for methods, apparatus and systems that can characterize thrombus formation for diagnosis and treatment purposes, and preferably in a substantially non-invasive manner.